Electric display devices provided with liquid crystal birefringent elements, commonly referred to as liquid crystal displays (LCD's), are characterized by their enhanced legibility, compact size and low electrical power requirements. LCD's are used extensively as optical display elements in digital watches and clocks, electronic calculators and electronic analytical equipment. LCD's, particularly those that display in color, are increasingly being used in place of cathode ray tubes (Brown's tube.) As advances in LCD design and fabrication techniques are realized LCD displays will increasingly become the output imaging device of choice in the near future.
Liquid crystal displays employ birefringent liquid crystals sandwiched between transparent sheets, glass or plastic sheets for example, to form liquid crystal elements. The transparent sheets receive Conductive coatings etched into character-forming segments through which an electric current is passed. An electric current passed through the liquid crystals is effective to cause them to become optically opaque. Thus, the segments, which may be segments of seven or more segment alpha-numeric displays, that are energized become optically opaque and prevent light from passing through the segments causing them to be visible. Display segments not energized by an electric current will remain clear and allow light to pass through.
Unlike other electro-optic output display devices, which include for example light emitting diode (LED) or fluorescent displays, liquid crystal elements themselves do not emit light. Thus, it is necessary to provide an external source of light to illuminate the LCD. This is done by reflecting ambient light off a reflective background or by providing a back lighting arrangement that projects illuminating light from the back side of the LCD through the liquid crystal elements.
Current back lighting devices, which are capable of achieving high and uniform illuminance, include so called common plane surface lighting bodies fabricated from single or plural laminated flat plates with painted, light scattering white dots covering one or more flat plate surfaces. The white dots are effective to reflect ambient light to provide back lighting to view the LCD. However, since ambient lighting may be dim or non existent altogether, these devices are often provided with light sources adapted to inject light beams into the flat plate edges to provide back lighting. The light beams travel through the flat plates at random angles with respect to the flat plate planar surfaces. If the beams strike a white dot the beams' trajectories are redireted to some degree perpendicular to the flat plane surfaces causing them to emit from the flat plates thereby backlighting the LCD elements.
Flat plate type common plane light-guiding panels tend to be quite thin with thin side edges for injecting the illuminating light. Thus, flat plate common plane light-guiding panels tend to be low in average luminance across the planar surface. Because of their low average surface luminance, common plane light guiding panels are most often used with monochromatic or black and white LCD's. Color LCD's require much greater average surface luminance making the use of common plane panels unsatisfactory.
Several modified light-guiding panel designs have been attempted to increase the average surface luminance. A wedge-shaped light-guiding panel, shown in FIG. 11, has been employed to provide increased surface luminance. Single wedge shaped light guiding panels provide increased luminance, however, the image area over which uniform luminance is achievable is quite small and unsuitable for the larger displays that are increasingly being demanded. The luminance can be increased by increasing the edge thickness to provide a greater coupling region for light coupling. This in turn requires that the incline angle be increased, the result being only a small increase in projected illuminating area while at the same time increasing the overall bulk of the display, making it unsuitable for many applications where a thin, cross-sectional component design is desirable.
A double-wedge light-guiding panel, which comprises, generally, two individual light-guiding panels disposed in optical communication with one another along abutting edges adjacent the wedges' acute angles, is shown in FIG. 12. The double-wedge configuration was proposed and fabricated to overcome the limitations of size and small projected area of illumination problematic with single wedge designs. Thus, the double wedge design provides a structure having a large image area and increased luminance (such as cold cathode discharge tube) at the outside edges of each of the two wedges opposite the wedges' acute angles.
The double-wedge configuration unfortunately has a defect described in the art as a brilliant line. A brilliant-line appears as a localized, longitudinally extending area of high intensity illumination adjacent the abutted edges of the two wedge structures.
At this point it becomes informative to discuss the various mechanisms of generation of the bright-line defect in prior art light-guiding panels.
A common characteristic among light-guiding panels having a bright-line defect is the presence of a discontinuity in the section profile of the light-guiding body. A discontinuity (that is, where a mathematical function defining the slope of the emitting surface contour is discontinuous at one or more locations) occurs, for example, at the intersection of two planes angularly displaced to one another and thus not coplanar.
In prior art light-guiding panels, the bright line defect generation mechanism depends on the configuration of the wedge shaped components employed to form the light-guiding panel. Looking the FIG. 7 there is shown a light-guiding panel generally indicated by the numeral 30. The panel 30 comprises light transmitting body 31 having a complex face 32 composed of a plurality of pairs of refractive planes symmetrically formed in the face 32 and designated by the numerals 34 and 35 and by planes defining the segments AO and OB, respectively. The planes 34 and 35 define a discontinuous emitting surface S.sub.E, having local discontinuities D.sub.1 and D.sub.2 where the planes AO and OB are caused to intersect. Above the surface S.sub.E is another refractive medium, air for example, having a refractive index less than the refractive index of the body 31.
The body 31 also includes symmetrically disposed light input surfaces 36 adapted to receive light from light sources 38. Reflective surface S.sub.R extends across the lower surface 39 of the body 31. Light scattering dots 40 are applied evenly on the surface 39. A reflective coating 41 is also disposed in optical contact with the surface S.sub.R, which may include a metallic mirror-like coating or a white paint coating. Thus, light produced by the light sources 38 is pumped into the body 31 through the input surface 36. Light rays entering at a trajectory below a plane parallel to the reflective surface S.sub.R will generally be incident on the surface S.sub.R and be reflected therefrom such that the ray is caused to be incident on the emitting surface S.sub.E, and emitted therefrom. Light rays entering at a trajectory above a plane horizontal to the surface S.sub.R will generally be incident directly at the upper surface 32 and refract therethrough. Some rays may be reflected by total internal reflection at the upper surface 32 and back to the surface S.sub.R where they are reflected back through the emitting surface S.sub.E.
FIG. 8 provides a vector illustration of the light passing through the surfaces AO and OB and the cause of the bright-line defect. Reference lines DOF and GOI, which are perpendicular to the faces AO and OB respectively and which pass through the point O, provide lines of reference for an analysis of light incident at the surface 34 and 35. Rays emitted from the plane face AO, have an angle smaller than the critical angle I.sub.AO. Where the light-guiding body is made of polymethyl methacrylate, a plastic material commonly used for fabricating light-guiding bodies, the critical angle is about 42.1.degree.. The incident rays within the angle COD are emitted from the point O and refracted within the angular displacement AOF. Simillarly, all rays incident upon the surface OB and having an angle smaller than the critical angle I.sub.OB, including the rays within the angle HOG, will be emitted from the surface OB within an angular displacement BOI. As can be seen illusratively in FIG. 8, there is a region of emitted light overlap defined within the angular portion FOI. The intensity of the rays at the point O within the region FOI is the total of the ray intensity from the plane faces AO and OB. The liminance (cd/m.sup.2) in the overlap region is greater than the average luminance of each face AO or OB individually and less than or equal to the arithmetic total of the components of the rays. The luminance is expressed mathematically by Formula III: ##EQU1## Where: LF.sub.OA =Luminance on the face of OA; and,
LF.sub.OB =Luminance on the face of OB. PA1 LC.sub.OB =Luminance on the concave OB
Thus, the luminance of the light-emitting face within .angle.FOI becomes remarkably greater than the luminance of faces OA or OB. The luminous strength of the line is dependent on the extent to which the plane of the surface defining OB is inclined with respect to the plane of the surface defining OA, the angular displacement being represented by the angle .alpha.. As .alpha. approaches 0.degree., that is, as the surfaces AO and OB tend toward a common plane defining AOE, the luminous strength of the bright-line tends toward the same strength as the luminous strength of surfaces AO and OB.
Other light-guiding panel configurations have similar bright-line generation mechanisms. A second common construction for light-guiding panels which also contains a bright-line defect is shown in FIG. 9 and is generally designated by the numeral 43. The light-guiding panel shown in FIG. 9 includes a light-guiding body 44 having an upper surface 45 defining an emitting surface S.sub.E. The emitting surface S.sub.E comprises a plane surface 46 defining line segment AO and an adjacent intersecting concave surface 47 defining arc segment OB. The arc segment OB may be described by a non-linear function such as a parabolic or hyperbolic function. The point of intersection O is a point of discontinuity between the function of the line segment AO and the arc OB and is the situs of a bright-line defect.
The mechanism of the generation of a brilliant line in the light-guiding panel shown in FIG. 9 is smilar to the defect generation mechanism of the panel shown in FIG. 7. The vector illustration of the bright-line defect of the light-guiding body shown in FIG. 9 is shown in FIG. 10. For the purpose of describing the defraction of light incident at the arc portion OB, reference orthogonal axis are included to assist in showing the trajectory of light being emitted from the surface S.sub.E. One of the axes has its point of origin at O and one of the axes, OB', is tangent to the arc OB. Line segment IOG, is perpendicular to the tangential plane OB' and intersects at the point O. All right rays incident on the arc portion OB' having an angle smaller than the critical angle I.sub.OB, (.angle.GOH), will be emitted as the refracted light within the angular portion FOI, As can be seen in FIG. 10, the refracted light in .angle.FOI includes components of light refracted from the plane OA and the concavity OB. The intensity of the rays at the point of intersection O is the total of rays from the faces OA and OB. The luminance, (cd/m.sup.2), of the bright line is greater than the average luminance of each face and equal to or smaller than the arithmetical total of the both as shown by the following Formila IV: ##EQU2## Where: LF.sub.OA =Luminance on the face of OA
Thus, the luminance of the light-emitting face within .angle.FOI becomes remarkably greater than that of plane face AO or concave face OB. The .angle.FOI is equal to the angle ,.alpha., of EOB between the plane OA and the tangential plane OB' at the point O which is the junction with the concavity OB. Therefore, as the angle .alpha. tends toward 0.degree. the angular displacement of .angle.FOI decreases, reducing the luminous strength of the bright-line defect. At .alpha.=0.degree. the plane defined by line segment OB' is caused to lie in the same plane as the plane defining line segment AO so as to define a plane defining line segment AOE. Where the light emitting surface S.sub.E comprising the plane OA is tangentially connected to the concavity OB at the junction point O, there will be no bright-line.
Attempts have been made to reduce the brilliant line defect in wedge configured light-guiding panels by causing the wedge to be truncated adjacent the acute angle edge and then abutting the truncated edges. This configuration is shown in FIG. 13. This arrangement, however, results in a dark-line defect adjacent the abutting edges E. Attempts to reduce the dark-line defect have included causing an arcuate valley to be disposed adjacent abutting portions of the truncated element device as shown in FIG. 14.
Heretofore, the prior art light-guiding panels employing some conbination of wedge shaped elements have not provided uniform illumination thought acceptable for such demanding deployments as back lighted LCD television screens.
In this circumstances, such symmetrical wedge-shaped light-guiding panel has not been practiced notwithstanding its merit of high luminance and broad image area.
The object of the present invention is to provide a novel light-guiding panel having high average luminance and a broad image area without the generation of a brilliant-line or dark-line defect, and to provide a new surface-lighting body by using such new light-guiding panel.
Moreover, the present invention is further directed to providing a compact surface lighting system by decreasing the weight and volume of the lighting body and to providing a structure to allow the disposal of an associated integrated circuit (IC) assembly into the space resulting from the minimization of the panel.